Method of writing to a scalable magnetoresistance random access memory element

ABSTRACT

A method to switch a scalable magnetoresistive memory cell including the steps of providing a magnetoresistive memory device sandwiched between a word line and a digit line so that current waveforms can be applied to the word and digit lines at various times to cause a magnetic field flux to rotate the effective magnetic moment vector of the device by approximately 180°. The magnetoresistive memory device includes N ferromagnetic layers that are anti-ferromagnetically coupled. N can be adjusted to change the magnetic switching volume of the device.

FIELD OF THE INVENTION

[0001] This invention relates to semiconductor memory devices.

[0002] More particularly, the present invention relates to semiconductorrandom access memory devices that utilize a magnetic field.

BACKGROUND OF THE INVENTION

[0003] Non-volatile memory devices are an extremely important componentin electronic systems. FLASH is the major non-volatile memory device inuse today. Typical non-volatile memory devices use charges trapped in afloating oxide layer to store information. Disadvantages of FLASH memoryinclude high voltage requirements and slow program and erase times.Also, FLASH memory has a poor write endurance of 10⁴-10⁶ cycles beforememory failure. In addition, to maintain reasonable data retention, thescaling of the gate oxide is restricted by the tunneling barrier seen bythe electrons. Hence, FLASH memory is limited in the dimensions to whichit can be scaled.

[0004] To overcome these shortcomings, magnetic memory devices are beingevaluated. One such device is magnetoresistive RAM (hereinafter referredto as “MRAM”). To be commercially practical, however, MRAM must havecomparable memory density to current memory technologies, be scalablefor future generations, operate at low voltages, have low powerconsumption, and have competitive read/write speeds.

[0005] For an MRAM device, the stability of the nonvolatile memorystate, the repeatability of the read/write cycles, and the memoryelement-to-element switching field uniformity are three of the mostimportant aspects of its design characteristics. A memory state in MRAMis not maintained by power, but rather by the direction of the magneticmoment vector. Storing data is accomplished by applying magnetic fieldsand causing a magnetic material in a MRAM device to be magnetized intoeither of two possible memory states. Recalling data is accomplished bysensing the resistive differences in the MRAM device between the twostates. The magnetic fields for writing are created by passing currentsthrough strip lines external to the magnetic structure or through themagnetic structures themselves.

[0006] As the lateral dimension of an MRAM device decreases, threeproblems occur. First, the switching field increases for a given shapeand film thickness, requiring a larger magnetic field to switch. Second,the total switching volume is reduced so that the energy barrier forreversal decreases. The energy barrier refers to the amount of energyneeded to switch the magnetic moment vector from one state to the other.The energy barrier determines the data retention and error rate of theMRAM device and unintended reversals can occur due to thermofluctuations(superparamagnetism) if the barrier is too small. A major problem withhaving a small energy barrier is that it becomes extremely difficult toselectively switch one MRAM device in an array. Selectablility allowsswitching without inadvertently switching other MRAM devices. Finally,because the switching field is produced by shape, the switching fieldbecomes more sensitive to shape variations as the MRAM device decreasesin size. With photolithography scaling becoming more difficult atsmaller dimensions, MRAM devices will have difficulty maintaining tightswitching distributions.

[0007] It would be highly advantageous, therefore, to remedy theforegoing and other deficiencies inherent in the prior art.

[0008] Accordingly, it is an object of the present invention to providea new and improved method of writing to a magnetoresistive random accessmemory device.

[0009] It is an object of the present invention to provide a new andimproved method of writing to a magnetoresistive random access memorydevice which is highly selectable.

[0010] It is another object of the present invention to provide a newand improved method of writing to a magnetoresistive random accessmemory device which has an improved error rate.

[0011] It is another object of the present invention to provide a newand improved method of writing to a magnetoresistive random accessmemory device which has a switching field that is less dependant onshape.

SUMMARY OF THE INVENTION

[0012] To achieve the objects and advantages specified above and others,a method of writing to a scalable magnetoresistive memory array isdisclosed. The memory array includes a number of scalablemagnetoresistive memory devices. For simplicity, we will look at how thewriting method applies to a single MRAM device, but it will beunderstood that the writing method applies to any number of MRAMdevices.

[0013] The MRAM device used to illustrate the writing method includes aword line and a digit line positioned adjacent to a magnetoresistivememory element. The magnetoresistive memory element includes a pinnedmagnetic region positioned adjacent to the digit line. A tunnelingbarrier is positioned on the pinned magnetic region. A free magneticregion is then positioned on the tunneling barrier and adjacent to theword line. In the preferred embodiment, the pinned magnetic region has aresultant magnetic moment vector that is fixed in a preferred direction.Also, in the preferred embodiment, the free magnetic region includessynthetic anti-ferromagnetic (hereinafter referred to as “SAF”) layermaterial. The synthetic anti-ferromagnetic layer material includes Nanti-ferromagnetically coupled layers of a ferromagnetic material, whereN is a whole number greater than or equal to two. The N layers define amagnetic switching volume that can be adjusted by changing N. In thepreferred embodiment, the N ferromagnetic layers areanti-ferromagnetically coupled by sandwiching an anti-ferromagneticcoupling spacer layer between each adjacent ferromagnetic layer.Further, each N layer has a moment adjusted to provide an optimizedwriting mode.

[0014] In the preferred embodiment, N is equal to two so that thesynthetic anti-ferromagnetic layer material is a tri-layer structure ofa ferromagnetic layer/anti-ferromagnetic coupling spacerlayer/ferromagnetic layer. The two ferromagnetic layers in the tri-layerstructure have magnetic moment vectors M₁ and M₂, respectively, and themagnetic moment vectors are usually oriented anti-parallel by thecoupling of the anti-ferromagnetic coupling spacer layer.Anti-ferromagnetic coupling is also generated by the magnetostaticfields of the layers in the MRAM structure. Therefore, the spacer layerneed not necessarily provide any additional antiferromagnetic couplingbeyond eliminating the ferromagnetic coupling between the two magneticlayers. More information as to the MRAM device used to illustrate thewriting method can be found in a copending U.S. patent applicationentitled “Magnetoresistance Random Access Memory for ImprovedScalability” filed of even date herewith, and incorporated herein byreference.

[0015] The magnetic moment vectors in the two ferromagnetic layers inthe MRAM device can have different thicknesses or material to provide aresultant magnetic moment vector given by ΔM=(M₂-M₁) and a sub-layermoment fractional balance ratio,$M_{br} = {\frac{\left( {M_{2} - M_{1}} \right)}{\left( {M_{2} + M_{1}} \right)} = {\frac{\Delta \quad M}{M_{total}}.}}$

[0016] The resultant magnetic moment vector of the tri-layer structureis free to rotate with an applied magnetic field. In zero field theresultant magnetic moment vector will be stable in a direction,determined by the magnetic anisotropy, that is either parallel oranti-parallel with respect to the resultant magnetic moment vector ofthe pinned reference layer. It will be understood that the term“resultant magnetic moment vector” is used only for purposes of thisdescription and for the case of totally balanced moments, the resultantmagnetic moment vector can be zero in the absence of a magnetic field.As described below, only the sub-layer magnetic moment vectors adjacentto the tunnel barrier determine the state of the memory.

[0017] The current through the MRAM device depends on the tunnelingmagnetoresistance, which is governed by the relative orientation of themagnetic moment vectors of the free and pinned layers directly adjacentto the tunneling barrier. If the magnetic moment vectors are parallel,then the MRAM device resistance is low and a voltage bias will induce alarger current through the device. This state is defined as a “1”. Ifthe magnetic moment vectors are anti-parallel, then the MRAM deviceresistance is high and an applied voltage bias will induce a smallercurrent through the device. This state is defined as a “0”. It will beunderstood that these definitions are arbitrary and could be reversed,but are used in this example for illustrative purposes. Thus, inmagnetoresistive memory, data storage is accomplished by applyingmagnetic fields that cause the magnetic moment vectors in the MRAMdevice to be orientated either one of parallel and anti-paralleldirections relative to the magnetic moment vector in the pinnedreference layer.

[0018] The method of writing to the scalable MRAM device relies on thephenomenon of “spin-flop” for a nearly balanced SAF tri-layer structure.Here, the term “nearly balanced” is defined such that the magnitude ofthe sub-layer moment fractional balance ratio is in the range0≦|M_(br)|≦0.1. The spin-flop phenomenon lowers the total magneticenergy in an applied field by rotating the magnetic moment vectors ofthe ferromagnetic layers so that they are nominally orthogonal to theapplied field direction but still predominantly anti-parallel to oneanother. The rotation, or flop, combined with a small deflection of eachferromagnetic magnetic moment vector in the direction of the appliedfield accounts for the decrease in total magnetic energy.

[0019] In general, using the flop phenomenon and a timed pulse sequence,the MRAM device can be written to using two distinct modes; a directwrite mode or a toggle write mode. These modes are achieved using thesame timed pulse sequence as will be described, but differ in the choiceof magnetic sub-layer moment and polarity and magnitude of the magneticfield applied.

[0020] Each writing method has its advantages. For example, when usingthe direct write mode, there is no need to determine the initial stateof the MRAM device because the state is only switched if the state beingwritten is different from the state that is stored. Although the directwriting method does not require knowledge of the state of the MRAMdevice before the writing sequence is initiated, it does requirechanging the polarity of both the word and digit line depending on whichstate is desired.

[0021] When using the toggle writing method, there is a need todetermine the initial state of the MRAM device before writing becausethe state will be switched every time the same polarity pulse sequenceis generated from both the word and digit lines. Thus, the toggle writemode works by reading the stored memory state and comparing that statewith the new state to be written. After comparison, the MRAM device isonly written to if the stored state and the new state are different.

[0022] The MRAM device is constructed such that the magnetic anisotropyaxis is ideally at a 45° angle to the word and digit lines. Hence, themagnetic moment vectors M₁ and M₂ are oriented in a preferred directionat a 45° angle to the directions of the word line and digit line at atime t₀. As an example of the writing method, to switch the state of theMRAM device using either a direct or toggle write, the following currentpulse sequence is used. At a time t₁, the word current is increased andM₁ and M₂ begin to rotate either clockwise or counterclockwise,depending on the direction of the word current, to align themselvesnominally orthogonal to the field direction due to the spin-flop effect.At a time t₂, the digit current is switched on. The digit current flowsin a direction such that M₁ and M₂ are further rotated in the samedirection as the rotation caused by the digit line magnetic field. Atthis point in time, both the word line current and the digit linecurrent are on, with M₁ and M₂ being nominally orthogonal to the netmagnetic field direction, which is 45° with respect to the currentlines.

[0023] It is important to realize that when only one current is on, themagnetic field will cause M₁ and M₂ to align nominally in a directionparallel to either the word line or digit line. However, if bothcurrents are on, then M₁ and M₂ will align nominally orthogonal to a 45°angle to the word line and digit line.

[0024] At a time t₃, the word line current is switched off, so that M₁and M₂ are being rotated only by the digit line magnetic field. At thispoint, M₁ and M₂ have generally been rotated past their hard-axisinstability points. At a time t₄, the digit line current is switched offand M₁ and M₂ will align along the preferred anisotropy axis. At thispoint in time, M₁ and M₂ have been rotated 180° and the MRAM device hasbeen switched. Thus, by sequentially switching the word and digitcurrents on and off, M₁ and M₂ of the MRAM device can be rotated by 180°so that the state of the device is switched.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The foregoing and further and more specific objects andadvantages of the instant invention will become readily apparent tothose skilled in the art from the following detailed description of apreferred embodiment thereof taken in conjunction with the followingdrawings:

[0026]FIG. 1 is a simplified sectional view of a magnetoresistive randomaccess memory device;

[0027]FIG. 2 is a simplified plan view of a magnetoresistive randomaccess memory device with word and digit lines;

[0028]FIG. 3 is a graph illustrating a simulation of the magnetic fieldamplitude combinations that produce the direct or toggle write mode inthe magnetoresistive random access memory device;

[0029]FIG. 4 is a graph illustrating the timing diagram of the wordcurrent and the digit current when both are turned on;

[0030]FIG. 5 is a diagram illustrating the rotation of the magneticmoment vectors for a magnetoresistive random access memory device forthe toggle write mode when writing a ‘1’ to a ‘0’;

[0031]FIG. 6 is a diagram illustrating the rotation of the magneticmoment vectors for a magnetoresistive random access memory device forthe toggle write mode when writing a ‘0’ to a ‘1’;

[0032]FIG. 7 is a graph illustrating the rotation of the magnetic momentvectors for a magnetoresistive random access memory device for thedirect write mode when writing a ‘1’ to a ‘0’;

[0033]FIG. 8 is a graph illustrating the rotation of the magnetic momentvectors for a magnetoresistive random access memory device for thedirect write mode when writing a ‘0’ to a state that is already a ‘0’;

[0034]FIG. 9 is a graph illustrating the timing diagram of the wordcurrent and the digit current when only the digit current is turned on;and

[0035]FIG. 10 is a graph illustrating the rotation of the magneticmoment vectors for a magnetoresistive random access memory device whenonly the digit current is turned on.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0036] Turn now to FIG. 1, which illustrates a simplified sectional viewof an MRAM array 3 in accordance with the present invention. In thisillustration, only a single magnetoresistive memory device 10 is shown,but it will be understood that MRAM array 3 consists of a number of MRAMdevices 10 and we are showing only one such device for simplicity indescribing the writing method.

[0037] MRAM device 10 is sandwiched therebetween a word line 20 and adigit line 30. Word line 20 and digit line 30 include conductivematerial such that a current can be passed therethrough. In thisillustration, word line 20 is positioned on top of MRAM device 10 anddigit line 30 is positioned on the bottom of MRAM device 10 and isdirected at a 90° angle to word line 20 (See FIG. 2).

[0038] MRAM device 10 includes a first magnetic region 15, a tunnelingbarrier 16, and a second magnetic region 17, wherein tunneling barrier16 is sandwiched therebetween first magnetic region 15 and secondmagnetic region 17. In the preferred embodiment, magnetic region 15includes a tri-layer structure 18, which has an anti-ferromagneticcoupling spacer layer 65 sandwiched therebetween two ferromagneticlayers 45 and 55. Anti-ferromagnetic coupling spacer layer 65 has athickness 86 and ferromagnetic layers 45 and 55 have thicknesses 41 and51, respectively. Further, magnetic region 17 has a tri-layer structure19, which has an anti-ferromagnetic coupling spacer layer 66 sandwichedtherebetween two ferromagnetic layers 46 and 56. Anti-ferromagneticcoupling spacer layer 66 has a thickness 87 and ferromagnetic layers 46and 56 have thicknesses 42 and 52, respectively.

[0039] Generally, anti-ferromagnetic coupling spacer layers 65 and 66include at least one of the elements Ru, Os, Re, Cr, Rh, Cu, orcombinations thereof. Further, ferromagnetic layers 45, 55, 46, and 56include at least one of elements Ni, Fe, Mn, Co, or combinationsthereof. Also, it will be understood that magnetic regions 15 and 17 caninclude synthetic anti-ferromagnetic layer material structures otherthan tri-layer structures and the use of tri-layer structures in thisembodiment is for illustrative purposes only. For example, one suchsynthetic anti-ferromagnetic layer material structure could include afive-layer stack of a ferromagnetic layer/anti-ferromagnetic couplingspacer layer/ferromagnetic layer/anti-ferromagnetic coupling spacerlayer/ferromagnetic layer structure.

[0040] Ferromagnetic layers 45 and 55 each have a magnetic moment vector57 and 53, respectively, that are usually held anti-parallel by couplingof the anti-ferromagnetic coupling spacer layer 65. Also, magneticregion 15 has a resultant magnetic moment vector 40 and magnetic region17 has a resultant magnetic moment vector 50. Resultant magnetic momentvectors 40 and 50 are oriented along an anisotropy easy-axis in adirection that is at an angle, preferably 45°, from word line 20 anddigit line 30 (See FIG. 2). Further, magnetic region 15 is a freeferromagnetic region, meaning that resultant magnetic moment vector 40is free to rotate in the presence of an applied magnetic field. Magneticregion 17 is a pinned ferromagnetic region, meaning that resultantmagnetic moment vector 50 is not free to rotate in the presence of amoderate applied magnetic field and is used as the reference layer.

[0041] While anti-ferromagnetic coupling layers are illustrated betweenthe two ferromagnetic layers in each tri-layer structure 18, it will beunderstood that the ferromagnetic layers could be anti-ferromagneticallycoupled through other means, such as magnetostatic fields or otherfeatures. For example, when the aspect ratio of a cell is reduced tofive or less, the ferromagnetic layers are anti-parallel coupled frommagnetostatic flux closure.

[0042] In the preferred embodiment, MRAM device 10 has tri-layerstructures 18 that have a length/width ratio in a range of 1 to 5 for anon-circular plan. However, we illustrate a plan that is circular (SeeFIG. 2). MRAM device 10 is circular in shape in the preferred embodimentto minimize the contribution to the switching field from shapeanisotropy and also because it is easier to use photolithographicprocessing to scale the device to smaller dimensions laterally. However,it will be understood that MRAM device 10 can have other shapes, such assquare, elliptical, rectangular, or diamond, but is illustrated as beingcircular for simplicity and improved performance.

[0043] Further, during fabrication of MRAM array 3, each succeedinglayer (i.e. 30, 55, 65, etc.) is deposited or otherwise formed insequence and each MRAM device 10 may be defined by selective deposition,photolithography processing, etching, etc. in any of the techniquesknown in the semiconductor industry. During deposition of at least theferromagnetic layers 45 and 55, a magnetic field is provided to set apreferred easy magnetic axis for this pair (induced anisotropy). Theprovided magnetic field creates a preferred anisotropy axis for magneticmoment vectors 53 and 57. The preferred axis is chosen to be at a 45°angle between word line 20 and digit line 30, as will be discussedpresently.

[0044] Turn now to FIG. 2, which illustrates a simplified plan view of aMRAM array 3 in accordance with the present invention. To simplify thedescription of MRAM device 10, all directions will be referenced to anx- and y-coordinate system 100 as shown and to a clockwise rotationdirection 94 and a counter-clockwise rotation direction 96. To furthersimplify the description, it is again assumed that N is equal to two sothat MRAM device 10 includes one tri-layer structure in region 15 withmagnetic moment vectors 53 and 57, as well as resultant magnetic momentvector 40. Also, only the magnetic moment vectors of region 15 areillustrated since they will be switched.

[0045] To illustrate how the writing methods work, it is assumed that apreferred anisotropy axis for magnetic moment vectors 53 and 57 isdirected at a 45° angle relative to the negative x- and negativey-directions and at a 45° angle relative to the positive x- and positivey-directions. As an example, FIG. 2 shows that magnetic moment vector 53is directed at a 45° angle relative to the negative x- and negativey-directions. Since magnetic moment vector 57 is generally orientedanti-parallel to magnetic moment vector 53, it is directed at a 45°angle relative to the positive x- and positive y-directions. Thisinitial orientation will be used to show examples of the writingmethods, as will be discussed presently.

[0046] In the preferred embodiment, a word current 60 is defined asbeing positive if flowing in a positive x-direction and a digit current70 is defined as being positive if flowing in a positive y-direction.The purpose of word line 20 and digit line 30 is to create a magneticfield within MRAM device 10. A positive word current 60 will induce acircumferential word magnetic field, H_(W) 80, and a positive digitcurrent 70 will induce a circumferential digit magnetic field, H_(D) 90.Since word line 20 is above MRAM device 10, in the plane of the element,H_(W) 80 will be applied to MRAM device 10 in the positive y-directionfor a positive word current 60. Similarly, since digit line 30 is belowMRAM device 10, in the plane of the element, H_(D) 90 will be applied toMRAM device 10 in the positive x-direction for a positive digit current70. It will be understood that the definitions for positive and negativecurrent flow are arbitrary and are defined here for illustrativepurposes. The effect of reversing the current flow is to change thedirection of the magnetic field induced within MRAM device 10. Thebehavior of a current induced magnetic field is well known to thoseskilled in the art and will not be elaborated upon further here.

[0047] Turn now to FIG. 3, which illustrates the simulated switchingbehavior of a SAF tri-layer structure. The simulation consists of twosingle domain magnetic layers that have close to the same moment (anearly balanced SAF) with an intrinsic anisotropy, are coupledanti-ferromagnetically, and whose magnetization dynamics are describedby the Landau-Lifshitz equation. The x-axis is the word line magneticfield amplitude in Oersteds, and the y-axis is the digit line magneticfield amplitude in Oersteds. The magnetic fields are applied in a pulsesequence 100 as shown in FIG. 4 wherein pulse sequence 100 includes wordcurrent 60 and digit current 70 as functions of time.

[0048] There are three regions of operation illustrated in FIG. 3. In aregion 92 there is no switching. For MRAM operation in a region 95, thedirect writing method is in effect. When using the direct writingmethod, there is no need to determine the initial state of the MRAMdevice because the state is only switched if the state being written isdifferent from the state that is stored. The selection of the writtenstate is determined by the direction of current in both word line 20 anddigit line 30. For example, if a ‘1’ is desired to be written, then thedirection of current in both lines will be positive. If a ‘1’ is alreadystored in the element and a ‘1’ is being written, then the final stateof the MRAM device will continue to be a ‘1’. Further, if a ‘0’ isstored and a ‘1’ is being written with positive currents, then the finalstate of the MRAM device will be a ‘1’. Similar results are obtainedwhen writing a ‘0’ by using negative currents in both the word and digitlines. Hence, either state can be programmed to the desired ‘1’ or ‘0’with the appropriate polarity of current pulses, regardless of itsinitial state. Throughout this disclosure, operation in region 95 willbe defined as “direct write mode”.

[0049] For MRAM operation in a region 97, the toggle writing method isin effect. When using the toggle writing method, there is a need todetermine the initial state of the MRAM device before writing becausethe state is switched every time the MRAM device is written to,regardless of the direction of the currents as long as the same polaritycurrent pulses are chosen for both word line 20 and digit line 30. Forexample, if a ‘1’ is initially stored then the state of the device willbe switched to a ‘0’ after one positive current pulse sequence is flowedthrough the word and digit lines. Repeating the positive current pulsesequence on the stored ‘0’ state returns it to a ‘1’. Thus, to be ableto write the memory element into the desired state, the initial state ofMRAM device 10 must first be read and compared to the state to bewritten. The reading and comparing may require additional logiccircuitry, including a buffer for storing information and a comparatorfor comparing memory states. MRAM device 10 is then written to only ifthe stored state and the state to be written are different. One of theadvantages of this method is that the power consumed is lowered becauseonly the differing bits are switched. An additional advantage of usingthe toggle writing method is that only uni-polar voltages are requiredand, consequently, smaller N-channel transistors can be used to drivethe MRAM device. Throughout this disclosure, operation in region 97 willbe defined as “toggle write mode”.

[0050] Both writing methods involve supplying currents in word line 20and digit line 30 such that magnetic moment vectors 53 and 57 can beoriented in one of two preferred directions as discussed previously. Tofully elucidate the two switching modes, specific examples describingthe time evolution of magnetic moment vectors 53, 57, and 40 are nowgiven.

[0051] Turn now to FIG. 5 which illustrates the toggle write mode forwriting a ‘1’ to a ‘0’ using pulse sequence 100. In this illustration attime t₀, magnetic moment vectors 53 and 57 are oriented in the preferreddirections as shown in FIG. 2. This orientation will be defined as a‘1’.

[0052] At a time t₁, a positive word current 60 is turned on, whichinduces H_(W) 80 to be directed in the positive y-direction. The effectof positive H_(W) 80 is to cause the nearly balanced anti-aligned MRAMtri-layer to “FLOP” and become oriented approximately 90° to the appliedfield direction. The finite anti-ferromagnetic exchange interactionbetween ferromagnetic layers 45 and 55 will allow magnetic momentvectors 53 and 57 to now deflect at a small angle toward the magneticfield direction and resultant magnetic moment vector 40 will subtend theangle between magnetic moment vectors 53 and 57 and will align withH_(W) 80. Hence, magnetic moment vector 53 is rotated in clockwisedirection 94. Since resultant magnetic moment vector 40 is the vectoraddition of magnetic moment vectors 53 and 57, magnetic moment vector 57is also rotated in clockwise direction 94.

[0053] At a time t₂, positive digit current 70 is turned on, whichinduces positive H_(D) 90. Consequently, resultant magnetic momentvector 40 is being simultaneously directed in the positive y-directionby H_(W) 80 and the positive x-direction by H_(D) 90, which has theeffect of causing effective magnetic moment vector 40 to further rotatein clockwise direction 94 until it is generally oriented at a 45° anglebetween the positive x- and positive y-directions. Consequently,magnetic moment vectors 53 and 57 will also further rotate in clockwisedirection 94.

[0054] At a time t₃, word current 60 is turned off so that now onlyH_(D) 90 is directing resultant magnetic moment vector 40, which willnow be oriented in the positive x-direction. Both magnetic momentvectors 53 and 57 will now generally be directed at angles passed theiranisotropy hard-axis instability points.

[0055] At a time t₄, digit current 70 is turned off so a magnetic fieldforce is not acting upon resultant magnetic moment vector 40.Consequently, magnetic moment vectors 53 and 57 will become oriented intheir nearest preferred directions to minimize the anisotropy energy. Inthis case, the preferred direction for magnetic moment vector 53 is at a45° angle relative to the positive y- and positive x-directions. Thispreferred direction is also 180° from the initial direction of magneticmoment vector 53 at time t₀ and is defined as ‘0’. Hence, MRAM device 10has been switched to a ‘0’. It will be understood that MRAM device 10could also be switched by rotating magnetic moment vectors 53, 57, and40 in counter clockwise direction 96 by using negative currents in bothword line 20 and digit line 30, but is shown otherwise for illustrativepurposes.

[0056] Turn now to FIG. 6 which illustrates the toggle write mode forwriting a ‘0’ to a ‘1’ using pulse sequence 100. Illustrated are themagnetic moment vectors 53 and 57, as well as resultant magnetic momentvector 40, at each of the times t₀, t₁, t₂, t₃, and t₄ as describedpreviously showing the ability to switch the state of MRAM device 10from ‘0’ to 1’ with the same current and magnetic field directions.Hence, the state of MRAM device 10 is written to with toggle write mode,which corresponds to region 97 in FIG. 3.

[0057] For the direct write mode, it is assumed that magnetic momentvector 53 is larger in magnitude than magnetic moment vector 57, so thatmagnetic moment vector 40 points in the same direction as magneticmoment vector 53, but has a smaller magnitude in zero field. Thisunbalanced moment allows the dipole energy, which tends to align thetotal moment with the applied field, to break the symmetry of the nearlybalanced SAF. Hence, switching can occur only in one direction for agiven polarity of current.

[0058] Turn now to FIG. 7 which illustrates an example of writing a ‘1’to a ‘0’ using the direct write mode using pulse sequence 100. Hereagain, the memory state is initially a ‘1’ with magnetic moment vector53 directed 45° with respect to the negative x- and negativey-directions and magnetic moment vector 57 directed 452 with respect tothe positive x- and positive y-directions. Following the pulse sequenceas described above with positive word current 60 and positive digitcurrent 70, the writing occurs in a similar manner as the toggle writemode as described previously. Note that the moments again ‘FLOP’ at atime t₁, but the resulting angle is canted from 90° due to theunbalanced moment and anisotropy. After time t₄, MRAM device 10 has beenswitched to the ‘0’ state with resultant magnetic moment 40 oriented ata 45° angle in the positive x- and positive y-directions as desired.Similar results are obtained when writing a ‘0’ to a ‘1’ only now withnegative word current 60 and negative digit current 70.

[0059] Turn now to FIG. 8 which illustrates an example of writing usingthe direct write mode when the new state is the same as the statealready stored. In this example, a ‘0’ is already stored in MRAM device10 and current pulse sequence 100 is now repeated to store a ‘0’.Magnetic moment vectors 53 and 57 attempt to “FLOP” at a time t₁, butbecause the unbalanced magnetic moment must work against the appliedmagnetic field, the rotation is diminished. Hence, there is anadditional energy barrier to rotate out of the reverse state. At timet₂, the dominant moment 53 is nearly aligned with the positive x-axisand less than 45° from its initial anisotropy direction. At a time t₃,the magnetic field is directed along the positive x-axis. Rather thanrotating further clockwise, the system now lowers its energy by changingthe SAF moment symmetry with respect to the applied field. The passivemoment 57 crosses the x-axis and the system stabilizes with the dominantmoment 53 returned to near its original direction. Therefore, at a timet₄ when the magnetic field is removed, and the state stored in MRAMdevice 10 will remain a ‘0’. This sequence illustrates the mechanism ofthe direct write mode shown as region 95 in FIG. 3. Hence, in thisconvention, to write a ‘0’ requires positive current in both word line60 and digit line 70 and, conversely, to write a ‘1’ negative current isrequired in both word line 60 and digit line 70.

[0060] If larger fields are applied, eventually the energy decreaseassociated with a flop and scissor exceeds the additional energy barriercreated by the dipole energy of the unbalanced moment which ispreventing a toggle event. At this point, a toggle event will occur andthe switching is described by region 97.

[0061] Region 95 in which the direct write mode applies can be expanded,i.e. toggle mode region 97 can be moved to higher magnetic fields, ifthe times t₃ and t₄ are equal or made as close to equal as possible. Inthis case, the magnetic field direction starts at 45° relative to thebit anisotropy axis when word current 60 turns on and then moves toparallel with the bit anisotropy axis when digit current 70 turns on.This example is similar to the typical magnetic field applicationsequence. However, now word current 60 and digit current 70 turn offsubstantially simultaneously, so that the magnetic field direction doesnot rotate any further. Therefore, the applied field must be largeenough so that the resultant magnetic moment vector 40 has already movedpast its hard-axis instability point with both word current 60 and digitcurrent 70 turned on. A toggle writing mode event is now less likely tooccur, since the magnetic field direction is now rotated only 45°,instead of 90° as before. An advantage of having substantiallycoincident fall times, t₃ and t₄, is that now there are no additionalrestrictions on the order of the field rise times t₁ and t₂. Thus, themagnetic fields can be turned on in any order or can also besubstantially coincident.

[0062] The writing methods described previously are highly selectivebecause only the MRAM device that has both word current 60 and digitcurrent 70 turned on between time t₂ and time t₃ will switch states.This feature is illustrated in FIGS. 9 and 10. FIG. 9 illustrates pulsesequence 100 when word current 60 is not turned on and digit current 70is turned on. FIG. 10 illustrates the corresponding behavior of thestate of MRAM device 10. At a time t₀, magnetic moment vectors 53 and57, as well as resultant magnetic moment vector 40, are oriented asdescribed in FIG. 2. In pulse sequence 100, digit current 70 is turnedon at a time t₁. During this time, H_(D) 90 will cause resultantmagnetic moment vector 40 to be directed in the positive x-direction.

[0063] Since word current 60 is never switched on, resultant magneticmoment vectors 53 and 57 are never rotated through their anisotropyhard-axis instability points. As a result, magnetic moment vectors 53and 57 will reorient themselves in the nearest preferred direction whendigit current 70 is turned off at a time t₃, which in this case is theinitial direction at time t₀. Hence, the state of MRAM device 10 is notswitched. It will be understood that the same result will occur if wordcurrent 60 is turned on at similar times described above and digitcurrent 70 is not turned on. This feature ensures that only one MRAMdevice in an array will be switched, while the other devices will remainin their initial states. As a result, unintentional switching is avoidedand the bit error rate is minimized.

[0064] Various changes and modifications to the embodiments hereinchosen for purposes of illustration will readily occur to those skilledin the art. To the extent that such modifications and variations do notdepart from the spirit of the invention, they are intended to beincluded within the scope thereof which is assessed only by a fairinterpretation of the following claims.

[0065] Having fully described the invention in such clear and conciseterms as to enable those skilled in the art to understand and practicethe same, the invention claimed is:

1. A method of switching a magnetoresistive memory device comprising thesteps of: providing a magnetoresistive memory element adjacent to afirst conductor and a second conductor wherein the magnetoresistivememory element includes a first magnetic region and a second magneticregion separated by a tunneling barrier, at least one of the first andsecond magnetic regions include N ferromagnetic material layers that areanti-ferromagnetically coupled, where N is an integer equal to at leasttwo, and where each layer has a magnetic moment adjusted to provide awriting mode, and also each of the first and second magnetic regions hasa magnetic moment vector adjacent to the tunneling barrier oriented in apreferred direction at a time t₀; turning on a first current flowthrough the first conductor at a time t₁; turning on a second currentflow through the second conductor at a time t₂; turning off the firstcurrent flow through the first conductor at a time t₃; and turning offthe second current flow through the second conductor at a time t₄ sothat one of the magnetic moment vectors adjacent to the tunnelingbarrier is oriented in a direction different from the initial preferreddirection at the time t₀.
 2. A method of switching a magnetoresistivememory device as claimed in claim 1 wherein a sub-layer magnetic momentfractional balance ratio of the one of the first and second magneticregions is in the range 0≦|M_(br)|≦0.1.
 3. A method of switching amagnetoresistive memory device as claimed in claim 1 wherein the timest₀, t₁, t₂, t₃, and t₄ are such that t₀<t₁<t₂<t₃<t₄.
 4. A method ofswitching a magnetoresistive memory device as claimed in claim 1 furtherincluding the step of orientating the first and second conductors at a90° angle relative to each other.
 5. A method of switching amagnetoresistive memory device as claimed in claim 1 further includingthe step of setting the preferred direction at the time t₀ to be at anon-zero angle to the first and second conductors.
 6. A method ofswitching a magnetoresistive memory device as claimed in claim 1 whereinthe steps of turning on the first and second current flows in the firstand second conductors, respectively, includes using a combined currentmagnitude that is large enough to cause the magnetoresistive memoryelement to switch.
 7. A method of switching a magnetoresistive memorydevice as claimed in claim 1 wherein the N layers of ferromagneticmaterial are separated by an anti-ferromagnetic coupling material toprovide the anti-ferromagnetic coupling.
 8. A method of switching amagnetoresistive memory device as claimed in claim 7 wherein the step ofproviding the magnetoresistive memory element includes using at leastone of Ru, Os, Re, Cr, Rh, and Cu or combinations and compounds thereofin the anti-ferromagnetic coupling material.
 9. A method of switching amagnetoresistive memory device as claimed in claim 7 wherein theanti-ferromagnetic coupling material has a thickness in a range of 4 Åto 30 Å.
 10. A method of switching a magnetoresistive memory device asclaimed in claim 1 wherein the step of providing the magnetoresistivememory element includes using one of Ni, Fe, Mn, Co, and combinationsthereof, in the ferromagnetic material.
 11. A method of switching amagnetoresistive memory device as claimed in claim 10 wherein the stepof providing the magnetoresistive memory element includes providing eachof the N layers of ferromagnetic material with a thickness between 15 Åand 100 Å.
 12. A method of switching a magnetoresistive memory device asclaimed in claim 1 including in addition a step of providing themagnetoresistive memory element with a substantially circular crosssection.
 13. A method of switching a magnetoresistive memory device asclaimed in claim 1 including in addition a step of scaling the volume byincreasing N such that the volume remains substantially constant orincreases and a magnetic moment fractional balance ratio of the one ofthe first and second magnetic regions remains constant as themagnetoresistive memory element is scaled laterally to smallerdimensions.
 14. A method of switching a magnetoresistive memory deviceas claimed in claim 1 including in addition a step of adjusting themagnetic moment of the N layers so that a magnetic field needed toswitch the magnetic moment vectors remains substantially constant as thedevice is scaled laterally to smaller dimensions.
 15. A method ofswitching a magnetoresistive memory device as claimed in claim 1 whereinthe step of providing the writing mode includes adjusting the moment ofeach layer of the N layers to provide a direct write mode at anoperating current such that the current in each of the first and secondconductors is pulsed with a positive polarity to write a state and thecurrent in both of the first and second conductors is pulsed with anegative polarity to reverse the state.
 16. A method of switching amagnetoresistive memory device as claimed in claim 15 wherein the timet₃ is approximately equal to t₄ so that the magnetoresistive memorydevice operates in the direct write mode at the operating current.
 17. Amethod of switching a magnetoresistive memory device as claimed in claim16 wherein the time t₁ is approximately equal to t₂ so that themagnetoresistive memory device operates in the direct write mode at theoperating current.
 18. A method of switching a magnetoresistive memorydevice as claimed in claim 1 wherein the step of providing the writingmode includes adjusting the moment of each layer of the N layers toprovide a toggle write mode at an operating current such that thecurrent in each of the first and second conductors is pulsed with a samepolarity to write a state and the current in both of the first andsecond conductors is pulsed with the same polarity to reverse the state.19. A method of switching a magnetoresistive memory device as claimed inclaim 18 including in addition steps of reading the magnetoresistivememory device to obtain stored information and comparing the storedinformation to program information to be written prior to the steps ofturning on and turning off the first and second current flows.
 20. Amethod of switching a magnetoresistive memory device as claimed in claim18 including in addition steps of providing the first and second currentflows by using unipolar direction currents.
 21. A method of switching amagnetoresistive memory device comprising the steps of: providing amagnetoresistive memory element adjacent to a first conductor and asecond conductor wherein the magnetoresistive memory element includes apinned magnetic region and a free magnetic region separated by atunneling barrier, the free magnetic region includes Nanti-ferromagnetically coupled layers of a ferromagnetic material, whereN is an integer greater than or equal to two, and where the N layersdefine a volume and each layer of the N layers has a moment adjusted toprovide a writing mode, and wherein a sub-layer magnetic momentfractional balance ratio of the one of the first and second magneticregions is in a range 0≦|M_(br)|≦0.1, and the free magnetic region has amagnetic moment vector adjacent to the tunneling barrier oriented in apreferred direction at a time t₀; and applying a word current pulse toone of the first and second conductors at a time t₁ and turning off theword current pulse at a time t₃ while additionally applying a digit linecurrent pulse to another of the first and second conductors at a time t₂and turning off the digit line current pulse at a time t₄, whereint₀<t₁<t₂<t₃<t₄ so that the magnetic moment vector of the free magneticregion adjacent to the tunneling barrier at the time t₄ is oriented in adirection different from the initial preferred direction at the time t₀.22. A method of switching a magnetoresistive memory device as claimed inclaim 21 wherein the step of providing the magnetoresistive memoryelement includes adjusting the magnetic moment of each layer of the Nlayers to provide a direct write mode at an operating current such thatthe current in each of the first and second conductors is pulsed with asame polarity to write a state and the current in each of the first andsecond conductors is pulsed with an opposite polarity to reverse thestate.
 23. A method of switching a magnetoresistive memory device asclaimed in claim 21 wherein the step of providing the magnetoresistivememory element includes adjusting the moment of each layer of the Nlayers to provide a toggle write mode at an operating current such thatthe current in each of the first and second conductors is pulsed with asame polarity to write a state and the current in each of the first andsecond conductors is pulsed with the same polarity to reverse the state.24. A method of switching a magnetoresistive memory device as claimed inclaim 23 including in addition steps of reading the magnetoresistivememory device to obtain stored information and comparing the storedinformation to program information to be written prior to the steps ofapplying the word current pulse and the digit line current pulse andturning off the word current pulse and the digit line current pulse. 25.A method of switching a magnetoresistive memory device as claimed inclaim 23 including in addition the steps of applying the word currentpulse and the digit line current pulse by using unipolar directioncurrents.
 26. A method of switching a magnetoresistive memory device asclaimed in claim 21 wherein the step of providing the magnetoresistiveelement including N layers of ferromagnetic material includes a step ofseparating the N layers by an anti-ferromagnetic coupling material toprovide the anti-ferromagnetic coupling.
 27. A method of switching amagnetoresistive memory device as claimed in claim 26 wherein theanti-ferromagnetic coupling material has a thickness in a range ofapproximately 4 Å to 30 Å.
 28. A method of switching a magnetoresistivememory device as claimed in claim 26 wherein the step of providing themagnetoresistive memory element includes using one of Ru, Os, Re, Cr,Rh, and Cu in the anti-ferromagnetic coupling material.
 29. A method ofswitching a magnetoresistive memory device as claimed in claim 21further including a step of orientating the first and second conductorsat approximately a 90° angle relative to each other.
 30. A method ofswitching a magnetoresistive memory device as claimed in claim 29further including a step of setting the preferred direction at the timet₀ to be at a non-zero angle to the first and second conductors.
 31. Amethod of switching a magnetoresistive memory device as claimed in claim21 wherein the step of applying the word current pulse and the digitline current pulse to the first and second conductors, includes using acurrent magnitude that is large enough to cause the magnetic momentvector of the free magnetic region adjacent to the tunneling barrier toswitch to a different direction relative to the orientation at the time₀.
 32. A method of switching a magnetoresistive memory device as claimedin claim 21 wherein the step of providing the magnetoresistive memoryelement includes using one of Ni, Fe, Mn, Co, and combinations thereof,in the layers of ferromagnetic material.
 33. A method of switching amagnetoresistive memory device as claimed in claim 32 wherein the stepof providing the magnetoresistive memory element includes forming eachlayer of the N layers with a thickness in a range of approximately 15 Åto 100 Å.
 34. A method of switching a magnetoresistive memory device asclaimed in claim 21 wherein the step of providing the magnetoresistivememory element includes providing an element with a substantiallycircular cross section.
 35. A method of switching a magnetoresistivememory device as claimed in claim 21 including in addition a step ofscaling the volume by increasing N such that the volume remainssubstantially constant or increases and a sub-layer magnetic momentfractional balance ratio remains constant as the magnetoresistive memoryelement is scaled laterally to smaller dimensions.
 36. A method ofswitching a magnetoresistive memory device as claimed in claim 21including in addition a step of adjusting the magnetic moment of the Nlayers so that a magnetic field needed to switch the magnetic momentvector of the free magnetic region remains substantially constant as thedevice is scaled laterally to smaller dimensions.
 37. A method ofswitching a magnetoresistive device comprising the steps of: providing amagnetoresistive device adjacent to a first conductor and a secondconductor wherein the magnetoresistive device includes a free magneticregion and a fixed magnetic region separated by a tunneling barrier, thefree magnetic region including an N layer synthetic anti-ferromagneticstructure that defines a volume, where N is an integer greater than orequal to two, the N layer synthetic anti-ferromagnetic structureincludes anti-ferromagnetically coupled ferromagnetic layers with anmagnetic moment vector adjacent the tunneling barrier oriented in apreferred direction at a time t₀, and the N layer syntheticanti-ferromagnetic structure is adjusted to provide a toggle write mode;reading an initial state of the magnetoresistive memory device andcomparing the initial state with a new state to be stored in themagnetoresistive memory device; and applying a word current pulse, onlyif the initial state and the new state to be stored are different, toone of the first and second conductors at a time t₁ and turning off theword current pulse at a time t₃ while additionally applying a digit linecurrent pulse to another of the first and second conductors at a time t₂and turning off the digit line current pulse at a time t₄.
 38. A methodof switching a magnetoresistive memory device as claimed in claim 37wherein the magnetic moment vector adjacent to the tunneling barrier isoriented at a different direction at the time t₄ relative to thepreferred direction at the time t₀, and t₀<t₁<t₂<t₃<t₄.
 39. A method ofswitching a magnetoresistive memory device as claimed in claim 37wherein the time t₃ is approximately equal to the time t₄ so that themagnetoresistive memory device operates in a direct write mode at anoperating current.
 40. A method of switching a magnetoresistive memorydevice as claimed in claim 39 wherein the time t₁ is approximately equalto the time t₂ so that the magnetoresistive memory device operates inthe direct write mode at the operating current.
 41. A method ofswitching a magnetoresistive memory device as claimed in claim 37further including a step of orientating the first and second conductorsat approximately a 90° angle relative to each other.
 42. A method ofswitching a magnetoresistive memory device as claimed in claim 37further including a step of setting the preferred direction at the timet₀ to be at a non-zero angle to the first and second conductors.
 43. Amethod of switching a magnetoresistive memory device as claimed in claim37 wherein the step of applying the word current pulse and the digitline current pulse to the first and second conductors includes using acurrent magnitude that is large enough to cause the magnetic momentvector of the N layer synthetic anti-ferromagnetic structure to orientin a direction different from the initial preferred direction at thetime t₀.
 44. A method of switching a magnetoresistive memory device asclaimed in claim 37 wherein the step of forming the magnetoresistivememory element includes using one of Ru, Os, Re, Cr, Rh, and Cu toprovide the anti-ferromagnetic coupling.
 45. A method of switching amagnetoresistive memory device as claimed in claim 37 wherein the stepof providing the magnetoresistive memory element includes providing anelement with a substantially circular cross section.
 46. A method ofswitching a magnetoresistive memory device as claimed in claim 37including in addition a step of scaling the volume by increasing N suchthat the volume remains substantially constant or increases and asub-layer moment fractional balance ratio remains constant as themagnetoresistive memory element is scaled laterally to smallerdimensions.
 47. A method of switching a magnetoresistive memory deviceas claimed in claim 37 including in addition a step of providing theword current pulse and the digit line current pulse by using unipolardirection currents.